It's on the tip of my tongue -- something related to a tool like "to pass through a vise". The meaning is along the lines of overcoming a character-defining struggle in your life; the kind a person only goes through once a lifetime.

2 Answers
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I don't know whether characters can be said to pass through a vise, exactly. Gripped maybe the more likely expression concerning vises, unless you want to invoke the idea of escape. But if other tools are in scope, how about forged in the crucible of X, where x is your momentous struggle? Anvils and furnaces are other instances where forging happens and mettle is proved, but I am too ignorant of smithery to say whether they are good tool metaphors.